1. Field of the Invention
The invention relates generally to control systems, and more particularly to systems and methods for controlling an electric submersible pump (ESP) used in downhole oil production, wherein the voltage of the power supplied to the ESP is optimized for the load that is actually experienced by the ESP.
2. Related Art
Oil is normally produced through wells that are drilled into oil-bearing geological formations. In many instances, the pressure in the formation is insufficient to force the oil to the surface of the well, so artificial lift systems such as ESP's are used to pump the oil out of the wells. Typically, the ESP motor for a particular well is selected so that the shaft power produced by the motor (when operated at the “nameplate” rating) matches the shaft load that is expected to be generated by the ESP pump as it lifts the oil out of the well. (The “nameplate” rating consists of a set of values for operating parameters—typically frequency, voltage, current, horsepower—that are assigned to a motor when it is manufactured and are identified on a nameplate for the motor.)
In many instances, however, the actual shaft load of the pump when it is installed in the well is not the expected load. If the mismatch is relatively small, it may not be significant. If, on the other hand, the mismatch is large, the efficiency of the motor may be significantly reduced. If it is determined before installation of the ESP that the shaft load of the pump will be different than expected, the motor can be re-rated from the nameplate rating so that the efficiency of the motor is optimized. It is more challenging to deal with the situation in which the motor has been installed and started before it is determined that the pump's load is not as expected. Because the shaft load is unknown, the motor cannot be rerated in the conventional manner. Further, it is difficult, expensive and time consuming to retrieve the ESP from the well and replace the motor with a more suitable design.
It would therefore be desirable to provide improved systems and methods for addressing mismatches between the nameplate rating of an ESP motor and the actual shaft load experienced by the ESP pump.